It is very important when electrically heating water-beds that the heating elements are very well electrically insulated, so that there is no possibility of current passing through the bed in any way.
European patent application EP-A-0,731,624 discloses a heating element for a water-bed of the sort as described above comprising a flexurally stiff metal support plate, on which a number of small ceramic heating plates are arranged with the aid of a thermally conductive adhesive. Electrical conductors are burnt into these small ceramic plates, which conductors serve as electrical resistance wire for generating heat. That surface of the support plate to which the small ceramic plates are attached is covered with an insulating plate of approximately the same dimensions as the support plate, and both plates are connected to one another in the region of the periphery in such a way that a space remains between the support plate and the insulating plate, in which the small ceramic plates are situated. In this known heating element, the burnt-in electrical conductors are attached by burning in a pasty mixture of small particles of noble metals, such as gold and silver.
This known heating element has various disadvantages. Small ceramic plates can easily break under flexural strain. If a small ceramic plate does break, the electrical conductor on the small plate is interrupted, with the result that the small plate will no longer be able to emit any heat. Thus in order to prevent the fragile small ceramic plates from breaking, the metal support plate to which these ceramic plates are attached has to have a high flexural stiffness, and to achieve this the support plate has to be relatively thick. However, the thicker the support plate, the more slowly the heat is transferred from the small ceramic plates, via the support plate, to the water of the water-bed. Another drawback is that the coefficient of expansion of the metal plate and the small ceramic plates is not identical. As a result, the metal plate will not expand to the same extent as the small ceramic plates when heated. Since it is necessary, in connection with the difference in expansion of the metal plate and the ceramic plates, to prevent excessive stresses on the fragile ceramic plates, the dimensions of the small ceramic plates should not be too large, and therefore, if the heating is still to be controlled within reasonable limits, it is necessary to use more than one small ceramic plate, for example six or more plates. If an plurality of small ceramic plates is used, each small ceramic heating plate has to be connected separately to the power supply. This is carried out here with the aid of a printed circuit board, which regulates the current to the various small ceramic plates. This known heating element thus comprises a very large number of components, so that its production costs are relatively high.
Other known heating systems make use of flexible metal wires, which are electrically insulated from the surroundings with the aid of a plastic material. Since it is always possible for moisture and air to appear inside the wiring, it has been found that this can cause faults, due to the possibility of local overheating.